316 THE HEAET. 



The apex of the heart is situated about three and a-half inches to the left of the 

 middle line, and in the fifth intercostal space. The apex of the left auricular appen- 

 dage is in the lower part of the second intercostal space or behind the third costal 

 cartilage, about an inch and a quarter from the left of the sternum. 



FIBROUS AND MUSCULAR, STRUCTURE OF THE HEART. 



The heart consists chiefly of muscular tissue ; but besides this and the 

 thin membranes investing its surface and lining its cavities, there enter 

 into the formation of its wall, numerous blood-vessels, absorbents, and 

 nerves, together with more or less fat and some areolar tissue. 



THE FIBROUS TISSUE belonging to the heart, besides what enters into the 

 structure of the different valves and the chordae tendinese, is found princi- 

 pally surrounding the auriculo- ventricular and great arterial orifices. When 

 we view the base of the heart so placed that the two auriculo-ventricular 

 orifices, which are separated only by the upper edge of the septum ventri- 

 culorum, are side by eide, instead of the right being somewhat in front of 

 the left, as is the case dm ing life, the aortic opening is seen to occupy a 

 position between and in front of them, and to have the opening of the 

 pulmonary artery immediately in front of it. The wall of the aortic opening 

 is firmly blended opposite one of the semilunar valves with the forepart of 

 the right margin of the left auriculo-ventricular opening ; and opposite the 

 angle between the other two valves it is in close contact with the margin of 

 the right auriculo-ventricular openings. In the angle between the aortic and 

 two auriculo-ventricular openings there is found a small fibro-cartilaginous 

 mass, which in some large animals, as the ox and elephant, is replaced by a 

 piece of bone. From this nodule a thick process extends backwards 

 between the two auriculo-ventricular orifices beneath the septum auricu- 

 larum, and others pass forwards forming bands, one on each side of the 

 aortic opening. These processes form the bases of what have been elabo- 

 rately described by authors as the fibrous or tendinous rings of the auriculo- 

 ventricular openings. These rings, and others which are described as 

 bounding the arterial orifices, have had a great importance imputed to them 

 as being the tendons of origin of the ventricular muscular fibres, a view 

 which, however, from recent investigations to be presently noticed, appears 

 to be incorrect. The rings around the auriculo- ventricular orifices consist of 

 only a small quantity of loose, white, fibrous tissue, continuous with that 

 which is found in the segments of the valves, strengthened on the sides 

 next the septum by the processes from the fibro-cartilaginous nodule. The 

 rings of the arterial orifices have been described by authors, and also in the 

 previous editions of this work, as each formed by a fibrous band or zone, 

 one edge of which is even, and gives attachment to the muscular fasciculi 

 of the ventricle, whilst the other is scalloped into three deep semilunar 

 notches, and is firmly fixed to the middle coat of the large artery. This 

 scalloped margin is simply the line of junction of the endocardium with the 

 festooned line of attachment of the semilunar valves and termination of the 

 artery, strengthened however by areolar tissue. The fibres of the middle 

 coat of the artery also, opposite the sinuses of Valsalva, are not arranged 

 annularly as in other parts of the vessel, but diverge from between the 

 sinuses, and spread upwards and laterally on the walls of the vessel ; and 

 the attachment of the artery to the ventricle is principally effected by 

 fibrous tissue continuous with the middle coat of the artery and with the 

 fibrous tissue in the valves, which spreads out between the small fasciculi of 

 the muscular substance, and is firmly connected with it. 



